1. Field of the Invention
The invention relates generally to a brake control apparatus that controls braking forces that are applied to wheels of a vehicle, and, more specifically to a technology for protecting a pressure sensor included in the brake control apparatus.
2. Description of the Related Art
There is a hydraulic control apparatus that generates a hydraulic pressure corresponding to an operation amount of a brake pedal in a hydraulic circuit, and supplies the hydraulic pressure generated in the hydraulic circuit to wheel cylinders, thereby applying braking forces to wheels of a vehicle. Also, there is a hydraulic control apparatus that is provided with an actuator which includes a pair of electromagnetically-controlled valves that are used to increase or decrease the pressures in wheel cylinders fitted to respective wheels, and an electronic control unit that controls the actuator. With the hydraulic control apparatus, an amount by which a brake pedal is operated by a driver is detected by, for example, a sensor, and translated into an electric signal. Then, the electric signal indicating the operation amount of the brake pedal is transmitted to the electronic control unit. Then, the electronic control unit controls the electromagnetically-controlled valves used to increase or decrease the hydraulic pressures in the wheel cylinders, whereby the hydraulic pressures in the wheel cylinders fitted to the four respective wheels of the vehicle are controlled individually in the optimum manner. As a result, it is possible to secure high levels of running stability and safety. A system in which an operation performed by a driver is translated into an electric signal and braking forces are controlled based on the electric signal is generally called a “brake-by-wire” system.
Japanese Patent Application Publication No. 2006-123889 (JP-A-2006-123889) describes a hydraulic brake control apparatus which is an example of the hydraulic control apparatus described above. When detecting a certain type of malfunction, the hydraulic brake control apparatus described in JP-A-2006-123889 closes a partition valve, which separates a front wheel-side system from a rear wheel-side system, to provide communication between a hydraulic booster and the wheel cylinders, and performs a braking operation directly using the hydraulic pressure generated in response to the operation of the brake pedal performed by the driver.
The hydraulic brake control apparatus described in JP-A-2006-123889 includes a pressure sensor that detects the pressure in the hydraulic circuit. When receiving an excessively-high pressure, a commonly-used pressure sensor may fail to operate properly. Therefore, in the above-described hydraulic brake control apparatus that has a mode in which a hydraulic pressure is generated, directly in the wheel cylinders in response to a brake pedal operation performed by the driver, the hydraulic pressure may become considerably high if the driver performs an operation in a certain manner. Therefore, a pressure sensor that withstands a higher pressure needs to be employed in the hydraulic brake control apparatus described in JP-A-2006-123889. Accordingly, such hydraulic brake control apparatus reduces a degree of flexibility in designing a pressure sensor and causes cost increases.
In order to address these problems, Japanese Patent Application Publication No. JP-2006-199089 (JP-A-2006-199089) describes a brake control apparatus for a two-wheeled motor vehicle. With the brake control apparatus, when the pressure of hydraulic fluid exceeds the upper limit of permissible hydraulic pressure due to a brake operation, the hydraulic fluid is temporarily introduced into an accumulator of a relief valve unit. Thus, the pressure in a passage near a pressure sensor is decreased, and therefore the pressure sensor is protected.
However, in the brake control apparatus for a two-wheeled motor vehicle described in JP-A-2006-199089, a brake is applied to the front wheel by a brake lever and a brake is applied to the rear wheel by a brake pedal, and a hydraulic circuit for the front wheel and a hydraulic circuit for the rear wheel are separately formed. Therefore, relief valve units need to be provided in the respective hydraulic circuits, which increases the number of components. Also, if the permissible value varies with each pressure sensor, relief valves configured differently based on the permissible values need to be provided. This complicates the design of the brake control apparatus. In addition, there are various types of brake control apparatuses that use hydraulic pressure to apply brakes. Accordingly, various types of technologies for protecting pressure sensors are required.